Measurement of small pressures.



E. THOMSON.

MEASUREMENT OF SMALL PRESSURES. APPLICATION FILED JULY 31.1912.

1,190,044 v Patented July 4. 1916.

"\Mtness e's: v Inventor Elihu Thomson 7 V a h i I I 7 N TED ELI'HU THOMSON, or 'swAMPsc'oTT, MASSACHUSETTS, assmnon- T GENERAL ELECTRIC COMPANY, AXCOBPORATION OF NEW YORK. f

v mms'unnmnn'riofi smatnrnnssnnns. 1 1 190 044 Specification};LettersPatent. I qtgfitgdJflly'4,1916," Application filed Ju1y31, 1912, Serial No. 712,439. T0 all'whom itmag concer strncteduand arranged-for carrying out Be it known thatI, ELIHU Tnomsom a" invention. citizen of the United Statesgresidingiat The? conduit 1 mayre resent any duct Swampscott, county of Essex, State o onveyinga fluid, the pressure of which is -5. Massachusetts, have invented :eert'ain fto henoted as above atmospl1ere, or it may 55 and useful Imprmements in Measurement represent areeeptacle, the pressure in which V of Small, Pressures, of which the following is low and is jto be noted. Enteringthi's is I i L isa'speeifieationj I pressure-colleeting device which may be" i This invention relates to themeasurement? a. form of Pitot tube, as 2, the position of of ll zgagp gssnreg d al o th mei swhich may belt-hanged Within the duet ifto; urement. of flow, where the flow is of l'o'ivl desired, by-fiexible connections, say 3', If; l it I h case, itv is found that mere pressiu'ealoneis'to b measured witha none of the methods, heretofore in use, satout flow a' simple connection through the isfies the con(litionssufiieiently well where Walls of the receptacle is required. The the pressures area very smafll fraetion'of eonneetion 3 leads toone endyofani indieat- 65" an atmosphere, as, in the casefofgases pass'j ngxchamher, suchas a tube 4,01? glass or ingthrough "furnace ducts; e'ourse,wt e other, transparent material, to the-other end mercury manometer is entirely out of the of whichi's"connected asmall centrifugal, questiomas the disturbance oi the mercury fan 5 which isrdriven at variable speed by a" I column is ent-irely too small. ulsogwitht motor 6, the speed of which is under exact; a liquid much lessdensi-i, suehf'asWateror control hy a variable rheostat oi other oil etailmownIdensitgw Here agaii the means The motor has connectedwitlr" difi'e'renees are very difficult, o 'imte with it shaft an accurate speed indicator or a accuracy when small and my jc'omplicated 't'ael ometer fi; 'Any meansfor I aryingthe' r, with meniscus eflerts. Mo'reg er,- ,tl1e m rotation of the fan may be (Elll1'l l ,(l or 75 centage of error, where the ation of the any means for changing the pressure which, column, say isone or two; mil eters, must it develops during rotation, The object necessarily be large aml it 'i esirable to ottlie fan to enable one to produce go sometimes far below this-"differencein pressure whioh is greater or ,less'than that i measurement; for example, the pi-essure de iof'tho pressure to be measured,atuvill, and, 80'

veloped by ventilating fansfalthough ,the therefore, to produce a pressure ivhieh exvolume is very great, may be too: low to be aetlybalanees the 'uressure to be measured. a measured in auy'such Way,as, jalso is the The i'uletot' the fan needs only toibe a slow il0W"0f fluids within piposor duets, J smallhole Il(ltl';tllQ axis, asit' has no output. I The presentinvention providesthe means :No'ivit will'b e evident thatifflthe pressure 85 for measurement of the smallestpressures Within thefduet l is slightly above 'atmosand floii's and,'; at the. same time, is not? phere, -and theian is not rotating there will limited to those pressures and flows, but can-;'lie"a;lslow flow outward. through the tube 4. be applied to measure larger ones, in tliose g Ifl however, the fan be in motion. {and 40 which {would come withi-nithfrange, of a "increased-in rotation this flow is' .r0vei 'sed 90,

Watermanometer. The in; old llpro 'iose [at some poi! t' To;indicate,however,these is a zero method and consists inprodueing: Conditions, l-neeessary touii'range son' ea. pressure by known means such value thing isibl iyithin the glass t'lllie{f:ii1 l;,fmi-. as to balance the pressu 3, to be ueas.ured;,, this purpps'eI 'iai'e made ,i lotjluh-9 and providing meams for indiik-atiug' the h nigh whiel pult's of smokesmay be-intr 95 ili ed at inte'r ils to the tube, :t'lfhis may efdoiie manuallyattwillor o'theiu'ise A ,ssvl.10, forexample, slich asga h'ottliginay c ontaina sniall a'uu iunt of ,ii'quauammonia, mam connected, as shoiirn, to theside' q value of such balancing; prisur l al provide a. suitable device for 7 time at which that balance i The accompanying (ii-airing, is i diagrain matie representation appati'atlls conslightly, a puff. of

v with the conditions of pressure branch 9, while 11 may be a rubber bulb containing a small amount of hydrochloric acid or fuming hitric.

acid, a few drops bemg sufficient. On

fall, it will be possible to so'balance the.

pressures that the entcringpuft will stand almost stationary or at least move with comparative slowness in either direction. At this time the readings of the tachometer or speed indicatoi are taken and by moving the mouth of the Iitot tube directed against the flow wit in the duct 1 to the dilferent positions read ngs can be taken at any part of the duct. By turning the endof the tube so as to be at right angles to the position shown the pressure within the duct can be balanced so that if it be, so-to-speak, a static pressure above atmosphere, this can be balanced. Then by turning it face to the flow, the additional pressure balanced and measured gives the rate of flow, it being understood that the pressure developed by the fan is in proportion to the square of the speed of revolution. 7

If it be desired to eliminate the actual pressure regardless of flow as a factor in the measurement, it is only necessary to connect, as is shown in dotted lines, 12, a tube from the interior of the duct or chamber to the entrance of the fan. It is, of course, necessary in using the apparatus to have calibrated the fan orat least to have made it of such known dimensions that under certain conditions or speeds it will develop certain pressure. For example, it may be driven at such a speed as to develop a measurable pressure by the water column manometer. Since it follows the law that the pressure is the square of the speed, the speeds corresponding to the lower pressures may easily be determined. It will be noticed too, that when the balance is obtained between the two pressures there is no duty on the fan, and no How, the measurement then being a strictly zero measurement. It only remains to say that the method which has been outlined can be applied where the pressures are below the atmosphere or where the effect is that 'of a suctionby reversing the function of the fan and making the exhaust end of the fan'communicate with the glass indicating tube. For very low pressure it is to be understood that the fan can be of very small dimensions, and that it it be of large dimensions, its speed must be proportionately low Where the measurements concern gases of different temperatures, corrections of course ever, as

mg a visible gas,

must be made for the expansion due to this temperature in arriving at the actual velocity of flow. but where the mass which passes a given point is concerned, irrespective of its state of expansion or contraction by variations in temperature, these. corrections will not be needed. It follows also that the reed ings are obtained as in relation to the atmospheric pressure, which; rising and tailing, may be supposed to affect the gases in the duct 1 as well as what is measured is, therefore, the difference from atm sphere. If, however, the receiver, or duct 1 was so situated that the gases there in existing at a low pressure were not affected by the barometric changes, as'where the vessel was practically closed, then it would be necessary to allow for the outside changes-in barometric pressure in arriving at anyproper estimate of absolute pressure in the duct or re ptacle. Inasmuch, how: the method is most applicable to such things as furnace ducts or ventilating ducts "the consideration of barometer need not enter into the case. 1

In accordance with the ,xrovisionsof the patent statutes, I have described the principle of operation of my invention, 11: gether witb'the apparatus which I new consider to represent the best embodiment there of; but I desire to have it understood that the apparatus shown is only illustrative, and that the invention can be carried out by other means.

the outside air alike, and

What I claim as new and desire to secure by, Letters Patent of the United States, is-- 1. The method of measuring small fluid pressures, which consistsin admitting said ressure to a transparent chamber,introdu& setting up in said chamber a counter-pressure sutlicient to check the movement of said visible gas and measuring the energy required to produce said counterpressure. v

2. Means for measuring small fluid pres. sures, comprising a pressure collecting de vice, a chamber connected therewith, :1 coun: ter-pressure producin said chamber, means ior operating thesarue, devices for indicating the value of such counter-pressure, and means for introducing a pull 0 visible gas into said chamber.

3. Means for measuring small fluid pressures, comprising a Pitot tube, it having one end connected therewith, a rotary fan blower connected with the other end of said tube, a motor for driving said fan, means for indicating the pressure-set up by device connectedwith I glass tube said fan blower, and means for injecting into said tube a puif oi smoke.

4.,vMeans for measuring small fluid pres-i sures, conun-ismg a litot tube, a glass tube having one end connected therewith, a re- "l5 wptaciles connected therewith for holding v hemicalsj adp, ted to produce a pufi of smok fglfsgid branc pipe and tube.

In Witness wliereof, I h aveher eunt o set lligghand this twenty-seventh day of July, ELIHU THOMSON, Witnesses:

JOHN A. McMAivUs, Jr..

FRANK G. Hu'r m. 

